The present invention relates to a photographic device, and more particularly to a focal point detector that can be used on a camera capable of achieving trimming photography to enhance the focusing accuracy or narrow the range of the distance to be found during trimming. Various focal point detectors have been put forward in the art so as to detect the focal points of optical instruments such as cameras, so that the results can be used for focusing. Among them, some focal point detectors making use of a light beam passing through a taking optical system or a part thereof are now widely used, primarily with single-lens reflex cameras. This is due to the advantages of the focal point detectors in that they are parallax-free irrespective of object distances; they can provide detection and correction at the time of focusing of errors in the production of taking lenses, etc. or in the driving of taking lenses for focusing; and given a light beam incident on them, they remain invariable in terms of focusing accuracy, even upon lens replacement. A typical focal point detector making use of a light beam passing through a taking optical system or a part thereof works on (1) the so-called phase correlation mode wherein light beams passing through different pupils of a taking lens are re-imaged through a pair of lenses to make use of a variation in the distance between the resulting images depending on defocusing, thereby achieving focal point detection, and on (2) the so-called contrast mode wherein the contrast of an image formed by a taking lens system is detected to find the position where it reaches a maximum, thereby achieving focal point detection.
The principle of the phase difference mode will now be explained with reference to FIG. 15 that is an optical path diagram. The arrangement shown in FIG. 15 is made up of a condenser lens 3 located in the vicinity of a predetermined image-formation plane (a predetermined focal plane or an equivalent film surface) 2, a pair of separator lenses 7 located in the rear of the condenser lens 3 and juxtaposed to each other with a gap large-enough to ensure the focusing accuracy, and a photoelectric conversion element array 8 located at a position where light beams emanating from the separator lenses 7 are to form an image.
When the taking lens 1 is in focus, an object image I is formed on the predetermined image-formation plane 2. This object image I is projected through the condenser lens 3 and compound-eye re-imaging lenses 7 on the secondary image-formation plane (the photoelectric conversion array 8) perpendicular to the optical axis of the taking lens 1, forming the first and second images I.sub.01 and I.sub.02. When an object image F is formed on the front focus or in front of the predetermined image-formation plane 2, it is projected through the lenses 3 and 7 on positions close and vertical to the optical axis of the taking lens 1, forming the first and second images F.sub.01 and F.sub.02. When an object image B is formed or the rear focus or in the rear of the predetermined image-formation plane 2, it is projected through the lenses 3 and 7 on positions farther away from and vertical to the optical axis of the taking lens 1, forming the first and second images B.sub.01 and B.sub.02. These first and second images are in the same direction, so that the distance between them can be detected to provide detection of in what state the taking lens 1 is in focus, inclusive of the front and rear focus amounts. More specifically, the light intensity distribution of the first and second images on the photoreceptor element array 8 is detected and then calculated or otherwise processed to find the distance between them. Many focal point detectors of the construction mentioned above have been disclosed in for example JP-A-55-118019, JP-A-58-106511 and JP-A-60-32012.
The principle of the contrast mode will then be explained with reference to the optical path diagrams of FIGS. 16 and 17. The arrangement shown in FIGS. 16 and 17 is built up of a condenser lens 3 located in the vicinity of a predetermined image-formation plane (a predetermined focal plane or an equivalent film surface) 2, a re-imaging lens 21 located in the rear of the condenser lens 3, and a photoelectric conversion element array 22 located at a position conjugate with respect to the predetermined image-formation plane 2. In FIG. 16 light beams passing through the taking lens 1 are shown to form an image on the predetermined image-formation plane 2, and in FIG. 17 light beams passing through the taking lens 1 are shown to form an image on a position in front of the predetermined image-formation plane 2 (or on the front focus). The image on the photoelectric conversion element array 22, as shown in FIG. 16, is well focused or of high contrast, and the image on the photoelectric conversion element array 22, as shown in FIG. 17, is poorly focused or of low contrast. If the taking lens system 1 is constantly moved in the direction of increasing contrast, it can then be moved to the focused position to achieving focusing. As disclosed in JP-A-63-127217, it is known that the contrast of an image formed through the taking optical system can be detected at two positions in the vicinity of the predetermined image-formation plane. It is also known that focal point detection can be achieved by moving the re-imaging lens.
On the other hand, many individuals have recently enjoyed full-textured pictures obtained by a certain trimming photography wherein 35-mm Leica size film with one frame size of 36 mm.times.12 mm (usually about 36 mm.times.24 mm) is exposed to light, and is then enlarged to about 7 times as large (usually to about 3.5 times as large (the so-called service size) and this is often desired by many individuals). Such photographs are called panoramic photographs. Many cameras capable of achieving panoramic-size photography are now available, and so are cameras that can easily be changed from normal size to panoramic size photography mode, and vice versa. These cameras are in greater demand than ever.
Never until now, however, is there proposed any TTL-phase difference AF (automatic focusing) system best suited for use on a camera capable of achieving trimming photography such as a normal/panoramic-size camera.
In the following description, the case where the same focus detector system is used to both normal and panoramic sizes.
Most of the chief subjects to be taken by those who use ordinary cameras are figures. For instance, consider the case of taking a souvenir photograph of some figures with a graceful range of mountains for the background, as depicted in FIG. 18. FIG. 19 represents a photographic range when some figures are mainly photographed in the normal size state, and FIG. 20 represents a photographic range when some figures are photographed in the panoramic size state with mountains for the background. The composition of FIG. 20 is exposed to light on film, as shown in FIG. 21. For panoramic photography, it is then necessary to shorten the focal length or move away from the subject, as compared with normal photography. The range of the distance to be found is shown by dotted lines in FIGS. 19, 20 and 21. The range of the distance to be found appears to be the same in size on film, but the size of the subject taken thereon varies largely. When the size of the subject coming within the range of the distance to be found is too large, it is difficult to detect the features (contrast) of the subject by the focus detector system, resulting in a lowering of the focusing accuracy. In addition, when nearby and distant objects come within the range of the distance to be found with a size reduction of the subject, as shown in FIG. 21, there is a phenomenon that distant and nearby objects coexist, which makes the focusing accuracy or probability (that is the probability that focusing is achievable) worse.
As already noted, it is generally necessary to make the focusing accuracy higher in the panoramic size state than in the normal size state, because the enlargement magnification in the former case is almost twice that in the latter. On the other hand, since panoramic-size photography is often used to take a photograph of more distant objects, as compared with normal-size photography, and so a lens having a shorter focal length is used, the demand on enlarging the range of the defocusing detected is less in panoramic-size photography than in normal-size photography.